1. Field of the Invention
The present invention relates to a gradient coil for MR devices, of the type having direct cooling by means of cooling lines that are embedded in the coil and through which a cooling agent flows.
2. Description of the Prior Art
In gradient coils for MR systems, significant heat arises due to the high electrical losses. This heat must be effectively dissipated by active cooling. Essential reasons for this dissipation of temperature include avoidance of an impermissible stress on the patient due to high temperature, as well as avoiding heating of the casting resin molding material used in the gradient coil system above its glass transition temperature. If the casting resin molding material is heated above its glass transition temperature, there will result drastic changes in the mechanical characteristics, as well as a possible formation of cracks in the molding material, or a dissolution of boundary surfaces and a resulting reduction of the TE starting voltage. The situation is made more critical by increasingly large differences in the thermal expansion of the materials used (copper, glass-fiber-reinforced plastic, casting resin) at higher temperatures. The maximum permissible temperature thereby represents an undesired limitation in the efficiency design.
Besides the possibility of cooling by providing hollow electrical lines, through which a cooling agent flows, as the coil windings, there is the possibility of dissipating the heat loss via two layers of two tightly wound cooling conduits made of metal or polyamide. Besides the less-than-optimal cooling due to the often poor heat conductivity of the conduit material used (temperature jump at the conduit wall), in the arrangements most often usedxe2x80x94which include, among others, structures wound in spiral fashionxe2x80x94long conduit lengths of approximately 50 meters result between the inlet and outlet. This results in a high loss of pressure in the lines, which, given cast-in connecting parts, can lead to leakages. Due to the sealed structure of the gradient coil system, leaking sections can be removed only with great difficulty or not at all, which is disadvantageous.
An object of the present invention is to provide an arrangement directly cooling a gradient coil that operates effectively with a simple design, while avoiding long cooling line lengths, and thereby enables a highly efficient construction of the gradient coil.
This object is inventively achieved in an arrangement wherein the cooling lines are disposed substantially parallel to one another, preferably running parallel to the axis of the gradient coil, and are combined to form heat exchanger modules whose individual cooling lines are coupled with one another so that the maximum length between the inlet and outlet of a module corresponds to twice the longitudinal length of the gradient coil.
By virtue of the inventive design in which the tube length through which coolant flows amounts only to a maximum of twice the coil longitudinal length, small pressure losses result, which also allows a lower inlet pressure to be used. This in turn also allows, among other things, a more advantageous design with thinner wall thicknesses, and thus with reduced heat resistance, so that the absorption of the heat from the surrounding heated areas of the gradient coil is improved. Due to the arrangement of the cooling lines in groups running parallel to one another, forming a module having an inlet and an outlet, the time-consuming winding of the cooling coil layers, is required in some known gradient coils, is also avoided.
The cooling lines of a module can be coupled at one end to a common cooling agent inlet and at the other end to a common cooling agent outlet. In an embodiment of this version the cooling agent inlet and/or the cooling agent outlet can terminate centrally into distribution channels whose cross-section is larger than that of the cooling lines. The inlet and the outlet are then located in alternating fashion at different end surfaces (coil ends) of the gradient coil, which can cause difficulties given the current manufacture of such gradient coils. This is because these coils, after being wound to the desired design are conventionally flooded with casting resin in a form from bottom to top, so that one frontal surface is sealed, and thus a cooling agent inlet or outlet leading the exterior can be arranged there only with difficulty.
In order to avoid these difficulties, in a further embodiment of the invention the cooling agent return is led back via a return line substantially parallel to the cooling lines, to the feed-in end surface of the gradient coil having the cooling agent inlet, and the return line has an expanded cross-section in relation to the primary cooling lines. Preferably, the return line has substantially oval or rectangular cross-section whose with a radial dimension corresponding to the radial dimension of the cooling agent lines, so that a module of cooling lines including such a return line has a uniform thickness. This allows the module to be disposed, for example, between successive winding layers of a gradient coil, particularly as a cylinder segment in the intermediate space between the windings of a gradient coil.
Alternatively, a portion of the cooling lines of a module, preferably exactly one-half of the cooling lines of a module, can be coupled with one another, at the side downstream from the inlet as return lines. In contrast to the two aforementioned exemplary embodiments, in which the cooling lines are each only as long as the longitudinal length of the gradient coil, in this embodiment effective cooling line lengths result that can be as much as twice the coil longitudinal length.